Connecting Ruby Logs and Traces

Connecting Ruby Logs and Traces

Trace correlation

In many cases, such as logging, it may be useful to correlate trace IDs to other events or data streams, for easier cross-referencing.

Logging in Rails applications

Automatic injection

For Rails applications using the default logger (ActiveSupport::TaggedLogging) or lograge, you can automatically enable trace correlation injection by setting the rails instrumentation configuration option log_injection to true or by setting environment variable DD_LOGS_INJECTION=true:

# config/initializers/datadog.rb
require 'ddtrace'

Datadog.configure do |c|
  c.use :rails, log_injection: true

Note: For lograge users who have also defined lograge.custom_options in an initializers/lograge.rb configuration file, because Rails loads initializers in alphabetical order, automatic trace correlation may not take effect, since initializers/datadog.rb would be overwritten by the initializers/lograge.rb initializer. To support automatic trace correlation with existing lograge.custom_options, use the Manual Lograge configuration below.

Manual injection


After setting up Lograge in a Rails application, manually modify the custom_options block in your environment configuration file (e.g. config/environments/production.rb) to add the trace IDs.

config.lograge.custom_options = lambda do |event|
  # Retrieves trace information for current thread
  correlation = Datadog.tracer.active_correlation

    # Adds IDs as tags to log output
    :dd => {
      # To preserve precision during JSON serialization, use strings for large numbers
      :trace_id => correlation.trace_id.to_s,
      :span_id => correlation.span_id.to_s,
      :env => correlation.env.to_s,
      :service => correlation.service.to_s,
      :version => correlation.version.to_s
    :ddsource => ["ruby"],
    :params => event.payload[:params].reject { |k| %w(controller action).include? k }

Rails applications that are configured with the default ActiveSupport::TaggedLogging logger can append correlation IDs as tags to log output. To enable Trace Correlation with ActiveSupport::TaggedLogging, in your Rails environment configuration file, add the following:

Rails.application.configure do
  config.log_tags = [proc { Datadog.tracer.active_correlation.to_s }]

# Given:
# DD_ENV = 'production' (The name of the environment your application is running in.)
# DD_SERVICE = 'billing-api' (Default service name of your application.)
# DD_VERSION = '2.5.17' (The version of your application.)

# Web requests will produce:
# [dd.env=production dd.service=billing-api dd.version=2.5.17 dd.trace_id=7110975754844687674 dd.span_id=7518426836986654206] Started GET "/articles" for at 2019-01-16 18:50:57 +0000
# [dd.env=production dd.service=billing-api dd.version=2.5.17 dd.trace_id=7110975754844687674 dd.span_id=7518426836986654206] Processing by ArticlesController#index as */*
# [dd.env=production dd.service=billing-api dd.version=2.5.17 dd.trace_id=7110975754844687674 dd.span_id=7518426836986654206]   Article Load (0.5ms)  SELECT "articles".* FROM "articles"
# [dd.env=production dd.service=billing-api dd.version=2.5.17 dd.trace_id=7110975754844687674 dd.span_id=7518426836986654206] Completed 200 OK in 7ms (Views: 5.5ms | ActiveRecord: 0.5ms)

Logging in Ruby applications

To add correlation IDs to your logger, add a log formatter which retrieves the correlation IDs with Datadog.tracer.active_correlation, then add them to the message.

To properly correlate with Datadog logging, be sure the following is present in the log message, in order as they appear:

  • dd.env=<ENV>: Where <ENV> is equal to Datadog.tracer.active_correlation.env. Omit if no environment is configured.
  • dd.service=<SERVICE>: Where <SERVICE> is equal to Datadog.tracer.active_correlation.service. Omit if no default service name is configured.
  • dd.version=<VERSION>: Where <VERSION> is equal to Datadog.tracer.active_correlation.version. Omit if no application version is configured.
  • dd.trace_id=<TRACE_ID>: Where <TRACE_ID> is equal to Datadog.tracer.active_correlation.trace_id or 0 if no trace is active during logging.
  • dd.span_id=<SPAN_ID>: Where <SPAN_ID> is equal to Datadog.tracer.active_correlation.span_id or 0 if no trace is active during logging.

By default, Datadog::Correlation::Identifier#to_s will return dd.env=<ENV> dd.service=<SERVICE> dd.version=<VERSION> dd.trace_id=<TRACE_ID> dd.span_id=<SPAN_ID>.

If a trace is not active and the application environment & version is not configured, it will return dd.trace_id=0 dd.span_id=0 dd.env= dd.version=.

An example of this in practice:

require 'ddtrace'
require 'logger'

ENV['DD_ENV'] = 'production'
ENV['DD_SERVICE'] = 'billing-api'
ENV['DD_VERSION'] = '2.5.17'

logger =
logger.progname = 'my_app'
logger.formatter  = proc do |severity, datetime, progname, msg|
  "[#{datetime}][#{progname}][#{severity}][#{Datadog.tracer.active_correlation}] #{msg}\n"

# When no trace is active
logger.warn('This is an untraced operation.')
# [2019-01-16 18:38:41 +0000][my_app][WARN][dd.env=production dd.service=billing-api dd.version=2.5.17 dd.trace_id=0 dd.span_id=0] This is an untraced operation.

# When a trace is active
Datadog.tracer.trace('my.operation') { logger.warn('This is a traced operation.') }
# [2019-01-16 18:38:41 +0000][my_app][WARN][dd.env=production dd.service=billing-api dd.version=2.5.17 dd.trace_id=8545847825299552251 dd.span_id=3711755234730770098] This is a traced operation.

Further Reading